The blue-excitable nucleic acid-binding fluorochromes, TOT0 and TO-PRO, were used to stain bacteria in marine plankton communities. Flow cytometric counts of cells stained with these dyes agreed well with counts made by epifluorescence microscopy of DAPI-stained cells. We have used these new dyes in a study of bacteria from various waters in the North Atlantic and the eastern Mediterranean. The results indicate a distinction among bacterial groups of different fluorescence intensities (apparent DNA content). At large spatial and temporal scales, about half of the variation in the percentage of bacteria with high apparent DNA content (termed group II bacteria) could be explained by the variation in chlorophyll. Further, the apparent mean DNA content of group II bacteria was also correlated with chlorophyll.

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Thirty-two strains ofEubacterium lentum and phenotypically similar anaerobic gram-positive bacilli were screened for intracellular bile salt 3a-and 12a-hydroxysteroid dehydrogenase (HSDHase) activities. These organisms were categorized into four groups: (A) those containing 12a-HSDHase only (10 strains), (B) those containing 3a-and 12a-HSDHase (13 strains), (C) those containing 3a-HSDHase only (2 strains), and (D) those devoid of any measurable HSDHase activity (7 strains). Of the respective four groups, 9/10, 13/13, 0/2, and 0/7 were like the neotype strain of E. lentum (ATCC 25559) in that they produced H2S in a triple sugar iron agar butt, reduced nitrate to nitrite, and weakly decomposed hydrogen peroxide. The other strains were variable for nitrate reduction and activity on hydrogen peroxide, but all the organisms in the first three categories (with one exception) were H2S producers (triple sugar iron agar butt) and all (with one exception) were designated E. lentum, whereas the organisms of category B were non-H2S producers (triple sugar iron agar butt). Five of these seven were not stimulated by arginine and are designated "phenotypically similar organisms." Thin-layer chromatography of extracted spent bacterial medium of four representative strains from each group grown in the presence of cholate revealed the presence of (A) .12-oxo product, (B) 12-oxo and 3-oxo products, (C) 3-oxo product, and (D) the absence of any of these products. The 12a-HSDHase of category B organisms was unstable unless 10'-M dithioerythritol was added to the buffer. With the exception of 3 out of 32 strains, there was a positive correlation between the production of measurable amounts of 12a-HSDHase and H2S production. Growth curves and the effect of arginine on growth and the production of 3a-and 12a-HSDHase were examined in representative strains of categories A, B, and C.